Photovoltaic devices using the photovoltaic effect in a semiconductor junction in a layer of an amorphous silicon derivative are known. The basic construction of such a photovoltaic device consists of a transparent electrode layer, a semiconductive layer (photoactive layer) and a back electrode layer deposited in this order on a transparent substrate such as glass.
On the other hand, as discussed in Japanese Laid-Open Patent Application No. 58-57756, a preprint of the Autumn Meeting of the Japanese Society of Applied Physics (1983) 25P-L-2, p. 351 and other Publications, the light incident surface of a transparent electrode layer has been texturized to a roughness of 0.1 um to 2.5 um to increase the optical path length of incident rays in order to improve the photoelectric conversion characteristics of a photovoltaic device.
It has been well known that optically generated charge carriers such as electrons and holes contributing to the generation of electricity are mainly generated in the non-doped or intrinsic region of the photoactive layer by the irradiation of light. Accordingly, in order to improve photoelectric conversion efficiency, it is necessary only to suppress the absorption of light by an impurity doped layer positioned closer to the light incident side than the intrinsic layer. To this end, the thickness of the impurity doped layer has been reduced to a few hundred angstroms. Also, as disclosed in Japanese Laid-Open Patent Application No. 57-95677, it has been proposed that the above described impurity doped layer on the light incident side be formed of, for example, amorphous silicon carbide having high light-transmissivity in order to obtain a window effect.
However, where the surface of the transparent electrode layer is texturized as described above, it is difficult to form the impurity doped layer on the transparent substrate side of the semiconductive layer uniformly in thickness, whereby a problem has occurred in that the semiconductor junction in the semiconductive layer tends to be imperfect.
On the other hand, as has been shown experimentally, metals of the platinum group such as Ir (metal elements Ru, Rh, Pd, Os, Ir and Pt belonging to Group VIII of the Periodic Table) may be used in the form of an oxide film to make a transparent electrode layer for the above described photovoltaic device. A sputtering method, an anodic oxidation method, a chemical transport reaction method and the like have been used for forming such an oxide film.
In the sputtering method, a material to be deposited is sputtered by heating or ion impact in a low-pressure atmosphere so as to deposit on a substrate, as disclosed in, for example, Physical Review B, Vol. 26, No. 2, p. 471. In the anodic oxidation method a metallic film is formed on a substrate by a suitable method and then is oxidized by an oxidation reaction when the substrate is used as an anode during electrolysis, as disclosed in, for example, "Technical Report of the Electronics and Communication Engineers of Japan, ED83-10, p. 1". In the chemical transport reaction method, for example, an Ir-oxide film is formed on a substrate by spraying O.sub.2 gas on metallic Ir in an atmosphere of about 1,400.degree. C. to form gaseous IrO.sub.3 having a high temperature and transferring the resulting gas to an atmosphere of about 1000.degree. C., where the gas is cooled to be transformed into solid IrO.sub.2 which deposits on the substrate, as disclosed in, for example, "Physical Review B, Vol. 24, No. 12, p. 7342.
However, each of the above described conventional methods has various kinds of problems. The sputtering method has problems in that the film-forming speed is small (1 .ANG./min or less). It is difficult to control the oxygen content in the film, since it is necessary to use a large-size target. The object on which the film is formed is also large-sized, thereby increasing manufacturing cost. The anodic oxidation method has problems in that it is difficult to increase the film thickness, it is difficult to oxidize the depth of the metallic film, and the like. The chemical transport reaction method has problems in that since it is necessary to carry it out in an atmosphere having high temperatures (800.degree. C. or more), the apparatus must be formed of heat-resistant materials, manufacturing facilities are expensive, and the like.